Recent years have seen increasing use of systems that use wireless communication to perform environmental measurements, monitoring, control, and the like. In wireless communication systems that perform environmental measurements, monitoring, control, and like, often the areas involved are relatively large, and often there are many obstructions to wireless communication within those areas, such cases it is beneficial to use a wireless communication network able to perform communication relayed by other devices to enable coverage within the area even when direct communication is not possible due to the environment, such as the installation locations of the receiving device and the transmitting device, the state of radio signals, and the like.
For this type of wireless communication network, one may consider a wireless communication network and that uses the ZigBee™ protocol (See, for example, Japanese Unexamined Patent Application Publication 2006-5928 and Japanese Unexamined Patent Application Publication 2006-42370). In the below, a wireless communication network that uses the protocol will be termed a “ZigBee network.”
A ZigBee network is a wireless communication network with a tree structure and is structured from the following three types of devices:
Device A (Node A)
There is only a single base layer (highest-level layer), known as the “coordinator,” which connects and communicates with lower-level devices. Moreover, this not only fulfills the role of the overall master station for the network as a whole, but also assigns the communication addresses (the network addresses) to the devices that are connected in a master/slave connection relationship (hereinafter termed simply a “master/slave relationship).
Device B (Node B)
Known as a “router,” this is on a level that is below that of the coordinator, and not only can connect and communicate with higher-level devices and lower-level devices, and receives commands from the coordinator, but also can connect to lower-level devices (routers and end devices) to fulfill a role as a local master station for those devices. Moreover, it assigns communication addresses (network addresses) to the devices connected in a master/slave relationship thereto. For example, a VAV controller, or the like, in an air conditioner controlling system would be a router.
Device C (Node C)
This is termed an “end device,” and exists at the terminus (the lowest level) of a branch on a network, and can connect and communicate with a higher-level device (a master device) that is connected in a master/slave relationship, but does not become a master station for any other device. For example, in an air conditioner controlling system, a sensor, such as a temperature sensor or a humidity sensor, would be an end device.
In this ZigBee network, when an end device sends a message to a higher-level device (a router or the coordinator), as the destination device, when that device is other than the master device (which would be a router) of that end device, then the transition of that message to the most appropriate router is performed through the master device, for example, performing a search for a route to the destination device.
Specifically, the router that is the most appropriate under the current radio signal conditions is selected from the mesh structure by the master device, through the specification of a route search when a message is transmitted from an end device, to thereby transmit the message. This makes it possible to relay the communication by searching for another communication route when one communication route becomes unusable for communication due to the effects of radio signal variability, such as multipath phasing.
Because of this, message transmission will not be cut off part way through during the outbound trip route from the end device to the destination device. Note multipath phasing is a phenomenon wherein the radio waves that are received are canceled out through phase differences that are produced between multiple communication radio signal reflection paths, preventing reception.
However, in the inbound path from the destination device to the end device, the return transmission destination is the end device, making it impossible to perform, at the destination device, a search for the route to the end device. Because of this, conventionally the transmission of a message back to the end device from the destination device is performed following the same route based on the tree route information for the communication address, where sometimes the message transmission is interrupted in process because of the state of the radio signals.
The present invention was created in order to solve this type of problem, and the object thereof is to provide a wireless communication system able to transmit reliably a message back from a destination device to an end device.